Switching power converters include a controller that controls the cycling of a power switch to regulate the delivery of power to a load. During a constant voltage control mode of operation, the controller controls the power switch cycling responsive to a feedback voltage signal derived from the output voltage delivered to a load. In a digital feedback loop, the feedback voltage signal is processed by a voltage sensing circuit including a comparator that drives a binary output signal responsive to whether the feedback signal is greater than or less than a reference voltage signal produced by a digital-to-analog converter (DAC). In an analog feedback loop, an error amplifier generates an error voltage responsive to the difference between the feedback voltage signal and the reference voltage signal.
Regardless of whether the controller has a digital or analog feedback loop, the controller determines a desired peak current for the current cycle of the power switch based upon the processing of the feedback voltage through the feedback loop. In particular, the controller monitors a sense voltage across a sense resistor in series with the power switch to determine whether the sense voltage indicates that the desired peak current has been obtained by comparing the sense voltage to a peak voltage that is proportional to the desired peak current. Once the controller determines that the sense voltage has reached the peak voltage, the controller switches off the power switch in the current power switch cycle.
The regulation of the output voltage through the cycling of the power switch according to each cycle's peak voltage determination is affected by the line voltage for the AC mains for providing the input power to the switching power converter. In particular, it is conventional for a controller to have to regulate the output voltage over a range of AC mains voltages. The particular AC line voltage depends upon country standards but is generally contained within a universal input range that covers roughly 90 VAC to 270 VAC. The AC line voltage is rectified through a diode bridge to produce a rectified input voltage having a magnitude that depends on the particular AC line voltage selected by the power utility. The rectified input voltage is smoothed through a bulk input capacitor but it is advantageous for the bulk input capacitor to be relatively small to keep manufacturing costs low, minimize harmonic distortion, and to lower the area demands on the printed circuit board on which the bulk input capacitor is mounted.
Given the small size of the bulk input capacitor, the rectified input voltage will have a sinusoidal profile that reaches a minimum value at each zero crossing for the AC line voltage. A rectified input voltage 100 is shown in FIG. 1A for a relatively high AC line voltage within the universal input range. Conversely, a rectified input voltage 105 results from a relatively low AC line voltage. The minimum voltage values for rectified input voltage 105 result in a low frequency line ripple in an output voltage 110 as shown in FIG. 1B while the switching power converter is subjected to a relatively heavy load while operating in a pulse width modulation mode of operation. Such low frequency line ripple will be present even for a relatively high AC line voltage during brown-out conditions.
Since such low frequency line ripple in the output voltage is undesirable, it is conventional for a controller to implement line ripple compensation in which the peak voltage for each power switch cycle is adjusted. In particular, the peak voltage depends not only upon the processing of the feedback voltage through the controller's feedback loop but also upon the switching period. Although the switching period is ideally constant during high-load pulse-width-modulation (PWM) of the power switch cycling, the actual switching period used in each switching cycle may deviate from the desired switching period value. For example, the desired switching period may expire but the secondary current in the secondary winding of the transformer (or the inductor current in a non-isolated switching power converter) has not yet reduced to zero. The controller must then wait until zero conduction current is achieved before the power switch may be cycled on (in a critical discontinuous conduction mode). Similarly, the reflected voltage on the primary winding will oscillate after the secondary current has reached zero. This reflected voltage may oscillate to values during a discontinuous conduction mode that would harm the power switch if it were cycled on at such a time. It is thus conventional to switch on the power switch during valleys in the reflected voltage oscillations in a technique known as valley-mode switching. Both valley-mode switching and critical discontinuous conduction mode operation may thus result in a difference between the desired switching period and the actual switching period used in a given cycle of the power switch.
Controllers with line ripple compensation monitor the difference between the desired switching period and the actual switching period in a preceding cycle of the power switch to adjust the peak voltage for a current cycle of the power switch. But such conventional line ripple compensation is only performed during heavy-load PWM operation of the power switch. However, heavy load conditions are no longer restricted to PWM modes of operation in modern switching power converters due to the development of direct charge techniques in which the switching power converter must directly charge a battery as opposed to providing a regulated DC output voltage that is then converted through a non-isolated switching power converter (e.g., a buck converter) within a mobile device to the proper voltage and current for charging the battery. Direct charging is advantageous because the mobile device no longer needs the non-isolated switching power converter for the charging of its battery, which lowers manufacturing costs. But direct charging involves the usage of pulse frequency modulation (PFM) of the power switch cycling at relatively heavy loads such that line ripple compensation is desirable. But conventional line ripple compensation is not applicable during PFM operation.
Accordingly, there is a need in the art for controllers for switching power converters with improved line ripple compensation that is applicable to both PWM and PFM modes of operation.